This invention relates to electrochemical processes and apparatus and is concerned with electrochemical processes and electrochemical cells employing particulate electrodes.
In general, electrochemical processes may be considered as being either cathodic processes or anodic processes depending on the electrode at which the technically important reaction occurs. Most cathodic processes involve either metal electrodeposition or electrolytic reduction of a constituent of the electrolyte in the presence of hydrogen formed at the cathode; in the former class of cathodic process are electroplating, electrorefining and electrowinning and in the latter class are the reduction of organic compounds and the production of caustic soda. Most anodic processes involve either the discharge of anions from solution at an essentially stable anode or the dissolution of the anode itself; in the former class of anodic process are processes for the production of chlorine and oxygen and in the latter class are processes for recovering valuable metal from scrap and the refining or purification of metals. Further details of industrial electrochemical processes are given in the book "Industrial Electrochemical Processes" edited by A. Kuhn and published by Elsevier in 1971.
The present invention is primarily, but not exclusively, concerned with cathodic processes, especially processes for electrowinning metals, and with particulate electrodes and electrochemical cells incorporating particulate electrodes which can be used in such processes. Electrowinning generally involves the recovery of a metal by deposition of the metal from aqueous leach liquors obtained by leaching an ore or concentrate with an aqueous electrolyte. Conventional electrowinning processes employ cells containing planar or grid-type anodes and planar cathodes. The anodes are generally insoluble and serve to conduct electricity to the electrolyte but in some instances the anode may consist of, for example, a corrodible matte anode. The nature of the cathode at the start of the process varies widely from process to process and may be, for example, a stainless steel, a titanium or an aluminium electrode on to which a thin layer of relatively pure metal is then deposited or a thin sheet of relatively pure metal (called a starting sheet and usually produced by deposition of the metal on to electrodes such as those mentioned above). During the electrowinning processes the metal to be recovered is deposited on the cathode which is permitted to grow to the desired thickness. The fully grown cathode is then removed from the cell for further processing. The electrolyte usually consists of an aqueous solution of one or more salts of the metal which solution is formulated so as to promote electrodeposition of the metal on the cathode in such form and purity as is desired and at acceptable power efficiencies. The cathodic current density is limited to relatively low values, e.g. 100-350 A/m.sup.2 for copper electrowinning, by the mass transfer effects at the cathode. In practice, operating conventional electrowinning cells above a certain critical current density would yield unacceptably rough, and therefore impure, cathodic products. The value of this critical current density is limited by the rate of the mass transfer processes transporting metal ions from the bulk of the electrolyte to the cathode and is a function of the following variables:
1. Concentration of metal ions in the electrolyte. PA0 2. Conductivity of the electrolyte. PA0 3. Concentration overpotential. PA0 4. Activation overpotential. PA0 5. Presence of impurities and solids. PA0 6. Presence of additives, such as levelling agents, brightness etc.
In recent years there have been described particulate electrodes which comprise a number of discrete particles consisting wholly or partially of electroconductive material and which, when the electrode is in use, are caused to move so as to be in intermittent contact either directly or through the agency of intermediate particles with at least one current conductor (which is often called the "current feeder" or "feeder electrode") by means of which an electric current is conducted to the particles. The electrical conductivity of the current conductor is generally not less than 10.sup.4 ohm.sup.-.sup.1 cm.sup.-.sup.1.
Particulate electrodes have been developed in a number of different forms. In one form, a mixture of the particles of the electrode and an electrolyte is pumped through a portion of the cell which contains the current conductor and in which the electrode reaction occurs, then around a circuit outside this portion of the cell, and is finally returned to the portion in which the electrode reaction occurs for further reaction. In another form, the particles of the electrode remain within the portion of the cell which contains the current conductor -- while an electrolyte only is passed through this portion and then around a circuit outside this portion. Included within this latter form are electrodes which, in operation, comprise a bed of particles through which there is an upward, evenly distributed, flow of the electrolyte; the particles become suspended in the electrolyte which flows at a rate such that the bed becomes expanded in volume, usually by more than 20 and an generally by 40 to 50 percent. The pattern of flow of electrolyte is arranged to be substantially constant through a horizontal cross-section within the bed of particles in order to achieve substantial uniformity of particle concentration in the horizontal plane. The terminology of fluidized beds has been applied to this form of particulate electrode and many of the properties of fluidized beds are evident in the behaviour of these so-called "fluidized bed electrodes". The high surface area of a fluidized bed electrode makes possible either the efficient electrolysis of dilute solutions or the use of a high current per unit volume of cell and per unit volume of electrolyte; for example, in copper deposition current densities up to 3000 A/m.sup.2 and more have been used experimentally. Particulate electrodes have been the subject of much research recently and examples of their formation, including fluidized bed electrodes, and their use in various electrochemical processes are disclosed in, for example, British Pat. Specification No. 1,194,181, U.S. Pat. Nos. 3,180,810, 3,527,617, 3,551,207 and 3,703,446, French Pat. No. 1,500,269 and Canadian Pat. No. 700,933.
In many electrochemical processes using particulate electrodes the electrode reaction involves deposition of ions on to the particles or dissolution of the material of the particles. In such processes the dimensions of the particles change with time and there may be provision for removal and replenishment of the particles. The choice of working conditions in such processes may be influenced by:
1. the need to avoid agglomeration of the particles of the electrode, paticularly the particles of a cathode on which electrodeposition of metal is taking place and particularly at diaphragms interposed between the anode and the cathode where agglomeration or plating has in the past been found to occur frequently;
2. the need to avoid excessive electrodeposition of the product of a cathode reaction on to the current conductor or agglomeration of the particles on the current conductor; and
3. the need to ensure a satisfactory rate of progress of the electrode reaction.
4. the need to obtain acceptable power efficiencies. Whilst the tendency towards agglomeration of particles may be reduced by increasing the rate of flow of electrolyte through the bed of particles, this increase in rate of flow in turn may reduce the rate of passage of charge from the current conductor to the particles of the electrode but increases the rate of passage of charge from the current conductor to the electrolyte and thus may increase the quantity of the product of the electrode reaction deposited at the current conductor.
It is an object of the present invention to provide an electrochemical process employing a particulate electrode in which the disadvantages of the particulate electrodes referred to above are ameliorated.
It is a further object of the invention to provide an improved method for the electrodeposition of metals.
It is a still further object of the invention to provide a novel particulate electrode, and a novel electrochemical cell incorporating said particulate electrode, suitable for use in electrochemical processes, including the electrowinning of metals such as copper, cobalt and nickel.